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Normally the body must be exposed to a foreign antigen before an antibody can be produced. This is not the case for the ABO blood group. Individuals with type A blood—without any prior exposure to incompatible blood—have preformed antibodies to the B antigen circulating in their blood plasma. These antibodies, referred to as anti-B antibodies, will cause agglutination and hemolysis if they ever encounter erythrocytes with B antigens. Similarly, an individual with type B blood has pre-formed anti-A antibodies. Individuals with type AB blood, which has both antigens, do not have preformed antibodies to either of these. People with type O blood lack antigens A and B on their erythrocytes, but both anti-A and anti-B antibodies circulate in their blood plasma.
The Rh blood group is classified according to the presence or absence of a second erythrocyte antigen identified as Rh. (It was first discovered in a type of primate known as a rhesus macaque, which is often used in research, because its blood is similar to that of humans.) Although dozens of Rh antigens have been identified, only one, designated D, is clinically important. Those who have the Rh D antigen present on their erythrocytes—about 85 percent of Americans—are described as Rh positive (Rh + ) and those who lack it are Rh negative (Rh − ). Note that the Rh group is distinct from the ABO group, so any individual, no matter their ABO blood type, may have or lack this Rh antigen. When identifying a patient’s blood type, the Rh group is designated by adding the word positive or negative to the ABO type. For example, A positive (A + ) means ABO group A blood with the Rh antigen present, and AB negative (AB − ) means ABO group AB blood without the Rh antigen.
[link] summarizes the distribution of the ABO and Rh blood types within the United States.
| Summary of ABO and Rh Blood Types within the United States | ||||
|---|---|---|---|---|
| Blood Type | African-Americans | Asian-Americans | Caucasian-Americans | Latino/Latina-Americans |
| A + | 24 | 27 | 33 | 29 |
| A − | 2 | 0.5 | 7 | 2 |
| B + | 18 | 25 | 9 | 9 |
| B − | 1 | 0.4 | 2 | 1 |
| AB + | 4 | 7 | 3 | 2 |
| AB − | 0.3 | 0.1 | 1 | 0.2 |
| O + | 47 | 39 | 37 | 53 |
| O − | 4 | 1 | 8 | 4 |
In contrast to the ABO group antibodies, which are preformed, antibodies to the Rh antigen are produced only in Rh − individuals after exposure to the antigen. This process, called sensitization, occurs following a transfusion with Rh-incompatible blood or, more commonly, with the birth of an Rh + baby to an Rh − mother. Problems are rare in a first pregnancy, since the baby’s Rh + cells rarely cross the placenta (the organ of gas and nutrient exchange between the baby and the mother). However, during or immediately after birth, the Rh − mother can be exposed to the baby’s Rh + cells ( [link] ). Research has shown that this occurs in about 13−14 percent of such pregnancies. After exposure, the mother’s immune system begins to generate anti-Rh antibodies. If the mother should then conceive another Rh + baby, the Rh antibodies she has produced can cross the placenta into the fetal bloodstream and destroy the fetal RBCs. This condition, known as hemolytic disease of the newborn (HDN) or erythroblastosis fetalis, may cause anemia in mild cases, but the agglutination and hemolysis can be so severe that without treatment the fetus may die in the womb or shortly after birth.
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